The present invention pertains to an ultrasonic air flowmeter for motor vehicles. More particularly, the invention relates to an ultrasonic air flowmeter including at least two transmitter-receivers which are provided to an intake passage of an internal combustion engine to measure the rate of an air flowing through the intake passage.
Heretofore, air flowmeters using a hot-wire a potentiometer or ultrasonic waves have been proposed as an air flowmeter to be equipped to motor vehicles. In the conventional air flowmeter using the ultrasonic waves, which generates Karman's vortex street in an air flow to measure an air flow rate, a sensor unit or a vortex street generator as well as the air flowmeters are provided in an air inlet passage. Therefore, pressure loss and the like may occur.
More specifically, the Karman's vortex street detection type ultrasonic air flowmeter which measures the flow velocity of an air by counting the number of Karman's vortexes generated for a certain unit time, has the following drawbacks. A laminar flow may often occur in a low speed range of the air flow, and therefore the vortexes are not easily formed. On the other hand, in a high speed range thereof, the vortex state becomes unstable.